Seiji and Shin first met when Shin was only 17 and Seiji was 27. At the time, Seiji was a cop looking out for Shin. Shin went on to become a policeman just like Seiji and one day he went to the station where Seiji worked in order to show him how big of an influence he's had on him. He got there only to find out that Seiji had left the force (to take over his inherited family's shop). They meet again 13 years later.
Feeling out of luck, Seiji utters the fateful words "maybe I should move on to men". Shin tells Seiji that he's been in love with him for so long and that if he wants to move on to men, Shin should be the one he moves on to.
Boku no Omawarisan